The present invention relates to an excitation control system provided to a synchronous machine such as a synchronous generator or a synchronous electric motor or the like, and more particularly, to an excitation control system for synchronous machines that are duplicated so as to increase the reliability.
In general, excitation control systems for synchronous machines have a basic function to perform automatic voltage regulation functions so as to control a terminal voltage of the synchronous machines to be constant, with ancillary functions being an overexcitation limit function and an underexcitation limit function so that the operation of the synchronous machine is stable and within the critical limits. Furthermore, there is also a control system provided with many complex functions such as secondary regulation functions so that the performance and the ineffective power are stable.
However, when there is the generation of a failure in such an excitation control system, a control abnormality generates and the control operation stops so that it is no longer possible to continue the operation of the synchronous machine. Accordingly, it is required that there be a high reliability of excitation control systems. In particular, in cases when there is a largecapacity synchronous generator provided to operating power facilities, it is necessary that there always be an extremely high reliability from the point of view of the public nature and the social importance of power supply.
Control systems for which such extremely high reliability is demanded are generally such that there is duplicated configuration of control systems so that the reliability of the control system itself is increased so as to prevent the generation of failures and so that it is possible to continue the operation of the machines that are the object of control even if a failure should generate.
Conventionally, phase control apparatus that generates thyristor arc pulses in synchronization with a phase angle of a voltage of an AC power are used in order to control an output voltage of a thyristor rectifier. When a high reliability is required of the phase control apparatus, one of the two control system is used as the regular system and the other is used as a standby system, with the regular system being normally used, and the standby system being used when there is a failure in the regular system.
In the phase control apparatus, the regular system and the standby system both have a phase detector that detects the phase angle of six phase components, a comparator that compares the output of said phase detectors and the phase control commands from a controller and outputs the pulse output conditions, and a pulse width limiter that outputs a pulse signal that is limited to a predetermined width when the pulse output conditions have become an arbitrary level out of two signal levels. Furthermore the control apparatus is provided with a switch to switch between the regular system and the standby system, and a failure detector to switch this switch when there is an abnormality in the pulse signals that are output from the regular system.
However, in the conventional phase control apparatus, there is a time delay from the detection of the abnormality in the regular system by the failure detector and the output of the switching signal, until the time that the switch actually switched from the regular system to the standby system and during this time, it is not possible to control the thyristor rectifier that there is the problem of the generation of swing in the DC component that is output from the thyristor.
In addition, when the six-component pulse signal output from the pulse limiter has stopped for some reason, it is still possible for the failure detector to detect the signal stop but detection is not possible when there is an abnormality in the pulse output condition signal or the six-component phase angle signal output from the comparator and the phase detector, and there is the problem that it is not possible to prevent control abnormalities in the thyristor rectifier.